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A method for measuring refractive index n of nanosize particles in the visible and near-IR spectral ranges is proposed. The method is based on comparing refractive index n colloid of a colloid solution of nanoparticles in several solvents with refractive indices n solvent of corresponding pure solvents and has an accuracy of ±2 × 10 -4 . Upconversion nanosize phosphors (UCNPs) are synthesized in the form of a β-NaYF 4 crystalline matrix doped with Yb 3+ , Er 3+ , and Tm 3+ rare earth ions. UCNPs have a doped core with a diameter of 40 ± 5 nm and undoped shell with a thickness of 3-5 nm. Synthesized nanocrystals possess intense photoluminescence in the blue, green, and red spectral ranges upon excitation by IR radiation with a wavelength of 977 nm. Using a spectroscopic refractometer, the dispersion of the refractive index of β-NaYF 4 /Yb 3+ /Er 3+ /Tm 3+ nanocrystals was measured for the first time in the spectral range of 450-1000 nm with an accuracy of ±2 × 10 -4 .

We carried out an experimental study of the use of nonlinear crystals ZnTe, GaP, GaSe, DAST, and LiNbO 3 and semiconductors GaAs and InAs for the rectification of femtosecond optical pulses. This conversion produces broadband terahertz pulses consisting of a single period of the field oscillation. Here, we discuss features of different types of the terahertz converters, their efficiency, and spectral range.